1. Field of the Invention
The present invention relates to semiconductor accelerometer switch for detecting acceleration affecting a device, in particular to a semiconductor accelerometer switch which can arbitrarily adjust the magnitude of acceleration to be detected.
2. Discussion of Background
Generally, when operation of an air bag system for an automobile is determined, acceleration is detected or measured in order to judge a collision.
Since high reliability is required for such a judgement, various countermeasures for safety have to be prepared against erroneous operation of an electronic circuit such as by electromagnetic interference (EMI), in case of emergency.
As an acceleration detecting apparatus for detecting acceleration of a moving object, various accelerometer sensors and various accelerometer switches were developed in the background of development of semiconductor micromachining technologies.
One type of the accelerometer sensors developed in the background of development in semiconductor micromachining technologies is a capacitance detection type.
FIGS. 39a and 39b are views for explaining a conventional accelerometer sensor disclosed in JP-A-7-120496, which explains a conventional accelerometer sensor of a capacitance-detection-type in detail.
FIG. 39a is a front view of the conventional accelerometer sensor; and FIG. 39b is a cross-sectional view of the conventional accelerometer sensor taken along a line 39b--39b in FIG. 39a.
In the Figures, numerical reference 1 designates a lower glass sheet; numerical reference 4 designates an upper glass sheet; numerical reference 9 designates a beam; numerical reference 10 designates a movable electrode; and numerical reference 15 designates a silicon substrate. The beam 9 and the movable electrode 10 are integrally formed by etching the silicon substrate 15.
Numerical reference 32 designates an upper fixed electrode; numerical reference 33 designates a lower fixed electrode; numerical reference 34 designates filler; and numerical reference 35 designates a connection terminal.
The accelerometer sensor in FIGS. 39a and 39b is formed such that the silicon substrate 15 is interposed between the lower glass sheet 1 and the upper glass sheet 4.
The silicon substrate 15 is thus interposed so that the movable electrode 10 is positioned between the upper fixed electrode 32 and the lower fixed electrode 33.
By such a structure, a first capacitor having a capacitance of C1 is composed of the upper fixed electrode 32 and the movable electrode 10 and a second capacitor having a capacitance of C2 is composed of the lower fixed electrode 33 and the movable electrode 10.
The movable electrode 10 is held by the beam 9 of a cantilever form, wherein when acceleration acts on the direction of the thickness of the silicon substrate 15 (the upward and downward directions in FIG. 39b), the movable electrode 10 is displaced by an inertial force. At this time, the direction of displacement depends on the direction of acceleration and the magnitude of displacement depends on the magnitude of acceleration.
By the displacement of the movable electrode 10, the capacitance C1 of the first capacitor and the capacitance C2 of the second capacitor are changed. Because the changes of the capacitance C1 of the first capacitor and the capacitance C2 of the second capacitor depend on the magnitude of displacement of the movable electrode 10, the acceleration acting on a device can be detected by detecting such changes of the capacitances.
However, the accelerometer sensor shown in FIGS. 39a and 39b generally had problems that a parasitic capacitance is difficult to reduce and EMI durability is low.
Air bag systems for automobiles are in a critical situation because EMI is generally used even though high reliability is required for the systems in consideration of a purpose of the systems.
Therefore, as a countermeasure for safety in consideration of erroneous operation of an electronic circuit, a mechanical accelerometer sensor for judging a collision is ordinarily provided in a front portion of an automobile in addition to the above-mentioned accelerometer sensor.
FIG. 40 is a view for explaining an example of the conventional accelerometer switch. In FIG. 40, numerical reference 28 designates a spherical weight; numerical reference 29 designates a magnet; and numerical references 30a and 30b designate contact points. When acceleration is not acting upon the device, the contact points 30a and 30b are opened. Further, under a condition that acceleration does not act on the device, the weight 28 is held by magnetic force of the magnet 29.
This accelerometer switch is operated by a principle for detecting when an inertia force acts on the weight 28. When an acceleration acting upon the weight is larger than force for holding the weight 28 by the magnet 29, the contacts 30a, 30b are pushed to mechanically close as a result of movement of the weight 28 in the direction of an arrow in FIG. 40.
FIG. 41 is an explanatory view for showing a structure of a conventional accelerometer switch in which a cross-sectional view thereof is shown.
A principle for detecting is the same as that in FIG. 40, wherein a weight moves in the direction of an arrow when acceleration is acted thereupon and touches both of a contact 30a and a contact 30b to there by electrically connect these.
A characteristic of this example is that mercury 31 is used as the weight, wherein this example is different from the case of FIG. 40 at a point that the weight is held by the gravity instead of by the magnet.
A conventional accelerometer sensor required an accelerometer switch generally located in a front portion of a vehicle and a collision was comprehensively judged by a signal from the accelerometer switch and a signal from the accelerometer sensor. However, although the conventional accelerometer switch shown in FIG. 40 had a simple structure, the magnet is used for holding a weight. Further, although the conventional accelerometer switch shown in FIG. 41 had a structure simpler than that of FIG. 40 and a cost thereof could be reduced, it is not preferable under a present situation of an environment impact which is seriously considered day by day because mercury is used as the weight.
Further, the conventional accelerometer switches shown in FIGS. 40 and 41 were not suitable for a cost reduction because these were manufactured by so-called machine-processing and the size of these was as large as several centimeters.
Further, in the conventional accelerometer switches shown in FIGS. 40 and 41, it was necessary to change the mass of weight in order to change a threshold of acceleration to be detected. Accordingly, there were problems not only that a design had to be changed in correspondence with the threshold of acceleration to be detected but also that general-purpose properties of accelerometer switch were closely limited.
Further, the conventional accelerometer switch shown in FIGS. 40 and 41 did not have a structure for maintaining a condition that the contacts 30 are connected. Therefore, there were problems that chattering was apt to occur; therefore the condition of the contacts was unstable; and an erroneous judgement by a whole system was apt to be caused lowering reliability of the accelerometer switch.